Acetabular prostheses generally consist of two separate components, an acetabular shell or cup and an acetabular insert or liner. The shell has a hemispherical shape and is affixed and embedded into a cavity formed in a natural acetabulum of a patient. The insert has a hemispherical shape to mate with an internal cavity of the shell. A low friction bearing surface is formed along a spherical cavity in the insert and is adapted to articulate with a femoral ball of a hip stem.
Typically, the shell is made of a biocompatible metal or metal alloy, and the insert is made of a polymer, such as ultrahigh molecular weight polyethylene. Regardless of the materials or geometries, these two components are generally locked together with the shell encompassing the external surface of the insert. Once the shell is embedded in bone of the natural acetabulum, the insert is ready to receive the femoral ball.
One disadvantage with acetabular prosthesis is that prior bipolar hip prostheses can experience impingement, subluxation, and even dislocation after being implanted in the patient. For instance, the spherical femoral ball of the hip stem can become dislocated from the acetabular component. This dislocation can occur from various reasons, such as trauma to the leg or abnormal twisting of the leg. In some instances, an additional surgical procedure is required to remedy dislocation of a prosthetic hip.
Due to the occurrence of impingement, subluxation, and other problems, it is desirable to have an acetabular insert that inhibits subluxation and dislocation of the femoral ball from the socket. In some designs, the insert is configured to have more than a hemispherical shape. In other words, the insert encloses and captures more than half of the femoral ball within the spherically shaped cavity of the insert itself. Prior patents illustrate an effort to design an insert with a spherically shaped cavity to capture the femoral ball.
U.S. Pat. No. 4,642,123 entitled “Ball and Joint Socket Bearing for Artificial Joint” to Noiles teaches, in one embodiment, an acetabular shell having two coaxial pin members and an acetabular liner having more than a hemisphere in one plane. The liner is rotatable within a spherical cavity of the shell about the coaxial pin members. In other embodiments, a retaining ring is used in conjunction with the shell and liner.
U.S. Pat. No. 4,871,368 entitled “Artificial Acetabulum” to Wagner teaches a one-piece acetabular liner that is mounted in a socket of an acetabular shell. The liner has an extension that extends in an asymmetric relation about a polar axis. This extension is laterally positioned and serves to prevent dislocation of the femoral head from the socket upon extreme deflections of the hip joint.
U.S. Pat. No. 5,002,577 entitled “Variable Position Acetabular Cup” to Bolesky et al. teaches an acetabular prosthesis having a shell, a liner, and an adaptor ring. The shell and liner have a symmetrical shape while the adaptor ring has a non-symmetrical shape. This adaptor can be mounted on the shell in a plurality of positions to change the position of the symmetrical liner after the shell is secured in the acetabulum.
U.S. Pat. No. 5,800,555 entitled “Acetabular Cup Bearing Liner” to Gray teaches a bearing liner formed with a rim that defines an opening to a concave bearing surface that encompasses more than a hemisphere. A channel is formed at the opening of the cavity to permit elastic deformation of the liner to allow the femoral ball to pass into the cavity. A locking component engages the liner to inhibit elastic deformation and capture the femoral ball.
It, therefore, would be advantageous to provide an acetabular prosthesis that provides an increase range of motion with respect to the femoral ball and reduces the occurrence of impingement, subluxation, and dislocation of the femoral ball from the acetabular insert.